The Fruitlet Size Distribution (FSD) Model, my 2024 experience…

As an introduction – why reinvent the wheel? – per LAURA HILLMANN AND TODD EINHORN, Michigan State University:

“Fruit set prediction models aim to produce timely estimates of fruitlet abscission after thinner applications to guide precision crop load management. The time to generate a prediction after an application is important to facilitate grower decisions to re-apply thinners while they are still efficacious, avoiding expensive hand thinning operations. The fruitlet growth rate (FGR) model, developed by Dr. Duane Greene, is a powerful tool that can accurately predict the percentage of fruitlets that will set in an orchard. Although an Excel data template and App are available to run the FGR model via computer and smartphone, respectively, adoption has been limited by the measurement-intensive procedure. A new approach, termed the ‘Fruitlet Size Distribution (FSD) Model’, described herein, was developed to produce predictions of apple fruit set comparable to the FGR model but achievable with less time investment. The principle underlying both models is the same: the relative growth rate or size of a fruitlet is compared to the most rapidly growing or largest fruitlet within the sample date to determine if it will abscise. Most predictions can be made within 8 days from thinner applications, though the duration of time depends on climatic, biological and horticultural factors. To optimize the FSD model we suggest beginning the model three days after the average fruitlet diameter of the orchard is 6 mm. Thus, the model partners well with thinning applications between bloom and 6 mm. For example, if a prediction can be achieved by 8 days, assuming an average growth of ~0.8 mm per day, then fruitlets will be ~ 12 mm if another application is needed; 12 mm fruitlets are very sensitive to many thinning chemistries.”

https://pacman.extension.org/2024/03/28/the-fruitlet-size-distribution-fsd-model-a-how-to-guide-2024-update/ Including a complete HOW-TO guide.

Let’s see if I followed the instructions?

✔ At the UMass Orchard in Belchertown, MA, in 2024 I tried the FSD model in two tall-spindle apple blocks, one Ambrosia, the other Cripps Pink ‘Maslin’ cv. Both were on G.41 rootstock. I flagged 8 representative trees in each block at bloom. 

✔ Marked five representative flowering spurs (clusters) per tree for a total of 40 clusters per block.

✔ Oh yea, I counted the total number of flower clusters on five of the marked trees, which ranged from 50 to 80 in Ambrosia, 90 to 170 (a lot) in Cripps Pink. 

✔ Over the course of three “sample” dates – May 22, 26, and 30 – in each block I collected – by either snipping, or pulling off, fruitlet stem included, see Figure ? – the fruitlets from each cluster, brought them inside, and weighed them, sitting down, nice. More on that in a minute.

✔ Data went right into the FSD Excel spreadsheet – can download from above link, have to enable Macros, it’s easier to do on a Mac than a PC, but that is another story  – and Voila! %FRUIT SET predicted in a nice chart! Figure 1 and 2.

Figure 1 - FSD prediction results for Cripps Pink

Figure 2 - FSD prediction results for Ambrosia

Humph. In terms of so “what good is it?,” if assuming a good bloom, I want the fruit set to be in the 10% range, equivalent to one apple for every two clusters? Right? That is up to you to set that target fruit set. From what I could see, we were on the right track, but I could also see for Ambrosia at least (Figure 2) it might be wise to put another chemical thinner spray on? Not sure if that happened, both blocks had a good (too many?) apples at the end. Yea, my follow through could use help. 🙂

Now for the minute I mentioned above, I was kind of impressed, particularly no measuring fruitlets in the orchard in the hot sun and bugs, just collect, and bring them into the comfort of air conditioning (no bugs if you keep the doors closed) where you individually (sitting down) throw them on an accurate scale (down to 0.01 grams accuracy), record, and hit ‘Start Calculations’ in the XLS FSD spreadsheet. Now, you can do this manually, but I had some money so I bought an Ohaus 'Scout' (STX222 AM) scale, and the USB interface, about $550. After some serious digging around in the PC (no Mac here, sorry) system and application settings, I got it to work! I had a little help from Laura in Einhorn’s MSU lab too. Put the fruitlet on scale, hit the scale enter button, and automatically record it in the spreadsheet. Figures 3 & 4. How cool! I was pretty psyched!

Figure 3 - Collected fruitlets (note with stems), scale, USB interface, and computer for fast weighing and recording of fruitlets

Figure 4 - Close up of OHAUS scale and Excel spreadsheet data entry

“Less time investment?” Yea, maybe, between collecting the fruitlets and weighing them, probably took 30 minutes. But I could do it by myself, two people not needed as with measuring and recording, so in terms of people hours, yes, definitely less. Did I mention the personal discomfort index was less too! That’s important to me… 🙂

So, I proposed MARKUSIM previously and now FSD to achieve one level – precision pruning, precision chemical thinning actually USING the FSD model, and hand thinning being other levels – of precision apple crop load management (PACMAN) for mere mortals! Me, not being one to leave it alone of course, and have to try it myself, also worked with OUTFIELD and VIVID-MACHINES in 2024, that’s upcoming. If you are thinking I will tie it all together at the end, dream on…I will leave it up to you to decide what you can do in terms of precision apple cropload management, one size does not fit all.

For complete instructions on how to use the FSD model, visit the PACMAN website.